This invention relates to epoxy resins used in fabrication of laminates such as are found in many electronic applications. More particularly, it relates to compositions which require little or none of the solvents generally required. Such solvents have been chosen particularly to dissolve curing agents such as dicyandiamide so that they can be uniformly distributed through the resin and thus can provide consistent laminate properties. However, the solvents used have been few in number and undesirable from an environmental standpoint. Consequently, improved curing agents are being developed which permit the use of less objectionable solvents. However, it would be more advantageous if no solvents were required or small amounts of less hazardous solvents were sufficient, thus eliminating or reducing worker exposure to solvents, reducing solvent emissions, and reducing or eliminating the generation of wastes. In addition, by utilizing low levels of environmentally conscious solvents or eliminating solvents, it is possible to generate higher quality prepregs and laminates due to the fact that residual high boiling point solvents will not be entrapped in the prepregs.
The basic requirements for curatives for epoxy based prepregs and laminates for printed circuit board applications and some cosmetic, structural, and non-structural applications are: 1) curatives which yield latent prepregs and compounded resins which do not significantly advance at 25.degree. C. to 35.degree. C. and 0% to 90% RH; 2) curatives which yield prepregs which show low or no advancement during storage at the above conditions, thus not requiring refrigeration during shipping and storage; 3) curatives which do not crystallize out of the epoxy resin varnish or prepreg, thus remaining homogeneous in properties and appearance; 4) curatives which do not result in the generation of volatile by-products which could result in voids being introduced during lamination; and 5) curatives which impart low viscosity and high gel times to the epoxy resin during prepregging and complete cure in a moderate time (15 min to 3 hours) at curing conditions ( 140.degree. to 180.degree. C.).
Dicyandiamide is well known as a curing agent for epoxy resin and as such possesses many desirable properties. It is also known to possess a serious deficiency in that it is soluble only in solvents which are undesirable, either because they are not suitable in most applications, such as water, or the solvents are relatively expensive and environmentally undesirable and high boiling, such as dimethylformamide, methylcellosolve, 1 -methoxy-2-propanol, N-methyl-2-pyrrolidinone, and the like. An object of this invention is to combine the benefits of reducing or eliminating the use of solvents while maintaining the characteristics of dicyandiamide cure of epoxy resins.
In U.S. Pat. No. 4,594,291 many epoxy resin compositions are described, some of which are said to require no solvents. While dicyandiamide is cited as a particularly suitable curing agent, no examples are given for its use. Bertram et al. claim it only as a curing agent for epoxy powder coatings since it is not suitable for liquid solvent free or environmentally conscious formulations because of its insolubility. Powder coating requires the use of specialized manufacturing equipment which is not applicable in the epoxy laminate industry.
Bertram et al. also teach the partial advancement of the epoxy resins in the presence of curing agents. They specifically teach that the compounded resin should be partially advanced to the extent that the melt viscosity has increased to a value which is at least about 20, preferably from about 25 to about 625, most preferably from about 50 to about 300, percent greater than the melt viscosity of the initial mixture components. The teachings in the patent demonstrate that the minimum temperature (mixing and degassing temperature) which is utilized is 100.degree. C., in some cases it may be 120.degree. to 130.degree. C., and in other cases the advancement should be carried out at 140.degree. to 150.degree. C. It is specific that the resin once compounded must be partially advanced prior to utilization as solvent free coatings, powder coatings, or laminating varnish. We have found that the increases in viscosity called for by Bertram, et al. are deleterious to the performance of our invention in that impregnation of the substrate is impeded by high viscosity.
We have now found that the substituted dicyandiamides of U.S. patent application Ser. No. 07/912,045 may be used in formulating epoxy resin compositions in which solvents are not required while maintaining the characteristics of dicyandiamide cure. In addition, we have found that the substituted dicyandiamides permit the utilization of low levels of environmentally conscious solvents to yield homogeneous resin varnishes while maintaining the characteristics of dicyandiamide cure. In addition, we have found that formulations based upon substituted dicyandiamides show no crystallization of the curative from the B-Staged resin, thus resulting in homogeneous properties throughout the prepreg and the lamination process.